This invention relates to one-part room temperature curable organopolysiloxane compositions having improved electrical properties, especially resistance to electrolytic corrosion.
In the prior art, one-part room temperature curable organopolysiloxane compositions are applied to circuit boards for electric and electronic applications and used as the moisture-proof and anti-fouling coat. A problem arises that the cured organopolysiloxane compositions experience a decline of insulation resistance upon application of voltage in a hot humid environment.
An object of the present invention is to provide a one-part room temperature curable organopolysiloxane composition which has eliminated the drawbacks of the prior art compositions, and cures into a product which is resistant to electrolytic corrosion in that no decline of insulation resistance occurs upon application of voltage in a hot humid environment.
It has been found that a room temperature curable organopolysiloxane composition comprising (a) an organopolysiloxane having a specific structure, (b) a alkenoxysilane or substituted alkenoxysilane or a partial hydrolyzate thereof, and (c) a siloxane or a partial hydrolyzate thereof cures into a product which does experience no decline of insulation resistance upon application of voltage in a hot humid environment and is resistant to electrolytic corrosion.
Accordingly, the present invention provides a room temperature curable organopolysiloxane composition comprising
(a) 100 parts by weight of an organopolysiloxane having the following general formula (I): 
xe2x80x83wherein R1 is hydrogen, a substituted or unsubstituted alkyl radical or a substituted or unsubstituted alkenyl radical, R2 is independently a substituted or unsubstituted monovalent hydrocarbon radical, X is an oxygen atom or a divalent hydrocarbon radical, a is 0, 1 or 2, and n is an integer of at least 10,
(b) 1 to 30 parts by weight of a substituted or unsubstituted alkenoxysilane having the following general formula (II): 
xe2x80x83wherein R3 is a substituted or unsubstituted monovalent hydrocarbon radical, R4 and R5 are each independently hydrogen or a substituted or unsubstituted monovalent hydrocarbon radical, and x is 3 or 4, or a partial hydrolyzate thereof, and
(c) 0.01 to 10 parts by weight of a siloxane containing per molecule at least one monovalent radical having the general formula (III): 
xe2x80x83wherein R6, R7, R8 and R9 each are hydrogen or a monovalent hydrocarbon radical, and y is an integer of 1 to 6, at least two silicon atoms, and at least three radicals selected from the group consisting of alkyl, alkoxy and alkenoxy radicals, or a partial hydrolyzate thereof.
Component (a) serving as the base in the composition of the invention is an organopolysiloxane having the following general formula (I). 
Herein, R1 is hydrogen, a substituted or unsubstituted alkyl radical, preferably having 1 to 10 carbon atoms, more preferably 1 to 8 carbon atoms, or a substituted or unsubstituted alkenyl radical, preferably having 2 to 10 carbon atoms, more preferably 2 to 8 carbon atoms. R2 which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon radical, preferably having 1 to 10 carbon atoms. X is an oxygen atom or a divalent hydrocarbon radical, preferably having 1 to 10 carbon atoms. The letter xe2x80x9caxe2x80x9d is 0, 1 or 2, and n is an integer of at least 10.
More particularly, of the radicals represented by R1, suitable alkyl radicals include, but are not limited to, methyl, ethyl, propyl, and butyl, and substituted ones of these alkyl radicals in which some or all of the hydrogen atoms are substituted with halogen atoms, cyano or the like, for example, halogenated alkyl radicals such as chloromethyl, trichloropropyl and trifluoropropyl and cyanoalkyl radicals such as 2-cyanoethyl, 3-cyanopropyl and 2-cyanobutyl. Of these, methyl and ethyl are preferred. Suitable alkenyl radicals include alkenyl radicals such as vinyl, allyl and isopropenyl, and substituted ones of these alkenyl radicals in which some or all of the hydrogen atoms are substituted with aryl radicals (e.g., phenyl), such as phenylethenyl. Of these, isopropenyl and vinyl are preferred. It is preferred that xe2x80x9caxe2x80x9d be 2 where R1 is hydrogen, and xe2x80x9caxe2x80x9d be 0 or 1 where R1 is an alkyl or alkenyl radical.
The hydrocarbon radicals represented by R2 include, but are not limited to, alkyl radicals such as methyl, ethyl, propyl, isopropyl, butyl, 2-ethylbutyl and octyl; cycloalkyl radicals such as cyclohexyl and cyclopentyl; alkenyl radicals such as vinyl and allyl; aryl radicals such as phenyl, tolyl, xylyl, naphthyl, biphenyl and phenanthryl; aralkyl radicals such as benzyl and phenylethyl; and substituted ones of the foregoing radicals in which some or all of the hydrogen atoms are substituted with halogen atoms, cyano or the like, for example, halogenated hydrocarbon radicals such as chloromethyl, trichloropropyl, trifluoropropyl, bromophenyl and chlorocyclohexyl, and cyano hydrocarbon radicals such as 2-cyanoethyl, 3-cyanopropyl and 2-cyanobutyl. Of these, methyl is preferred.
X is an oxygen atom or an alkylene radical such as methylene, ethylene and propylene, with the oxygen and ethylene being preferred.
Preferably, the organopolysiloxane is capped with a hydroxyl radical at either end of its molecular chain, especially with one hydroxyl radical at each end of its molecular chain.
In formula (1), n is an integer of 10 or greater. The organopolysiloxane preferably has a viscosity of about 25 to 1,000,000 centistokes (cSt) at 25xc2x0 C., and more preferably about 500 to 500,000 cSt at 25xc2x0 C.
Component (b) serving as a curing agent in the inventive composition is an alkenoxysilane or substituted alkenoxysilane having the following general formula (II) or a partial hydrolyzate thereof. 
Herein, R3 is a substituted or unsubstituted monovalent hydrocarbon radical, preferably having 1 to 10 carbon atoms. R4 and R5 which may be the same or different are hydrogen or substituted or unsubstituted monovalent hydrocarbon radicals, preferably having 1 to 10 carbon atoms. The letter x is 3 or 4.
More particularly, suitable hydrocarbon radicals represented by R3 include alkyl, alkenyl, aryl and other radicals as exemplified above for R2. The monovalent hydrocarbon radicals represented by R4 and R5 include alkyl, alkenyl and other radicals as exemplified above for R2.
Illustrative, non-limiting examples of the alkenoxysilane (b) include methyltrivinyloxysilane, methyltri(isopropenyloxy)silane, vinyltri(isopropenyloxy)silane, phenyltri(isopropenyloxy)silane, propyltri(isopropenyloxy) silane, tetra(isopropenyloxy)silane, methyltri(1-phenylethenyloxy)silane, methyltri(1-butenyloxy)silane, methyltri (1-methyl-1-propenyloxy)silane, methyltri(1,4-dimethyl-1,3-pentadienyloxy)silane and partial hydrolyzates thereof.
Usually, the alkenoxysilane (b) is added in an amount of 1 to 30 parts, preferably 3 to 20 parts, and more preferably 3 to 10 parts by weight per 100 parts by weight of the organopolysiloxane (a). Less than 1 part of component (b) effects insufficient crosslinking, failing to provide the desired rubbery elasticity in the cured state. More than 30 parts of component (b) entails an increased shrinkage factor upon curing, resulting in poor mechanical properties.
Component (c) serving as a curing catalyst in the inventive composition is a siloxane or a partial hydrolyzate thereof. The siloxane should contain per molecule at least one monovalent radical having the general formula (III), at least two silicon atoms, and at least three radicals selected from among alkyl, alkoxy and alkenoxy radicals. 
Herein R6, R7, R8 and R9 each are hydrogen or a monovalent hydrocarbon radical, preferably having 1 to 10 carbon atoms, and y is an integer of 1 to 6. Examples of the monovalent hydrocarbon radicals represented by R6, R7, R8 and R9 include those exemplified above for R2.
Preferably, the siloxane (c) has the following general formula (IV). 
Herein R6, R7, R8, R9, and y are as defined in formula (III). R10 which may be the same or different is hydrogen, a monovalent hydrocarbon radical, preferably having 1 to 10 carbon atoms, or a radical of the formula: 
wherein R11 is a substituted or unsubstituted monovalent hydrocarbon radical, preferably having. 1 to 10 carbon atoms, R12 is a monovalent hydrocarbon radical, preferably having 1 to 10 carbon atoms, and b is 0, 1 or 2. The letter z is an integer of 1 to 5.
Examples of the hydrocarbon radicals represented by R10, R11 and R12 include those exemplified above for R2. R12 is preferably selected from alkyl and alkenyl radicals.
Preferred examples of the siloxane (c) are given below. 
In the formulae, Me is methyl, Et is ethyl, Pr is propyl, Ph is phenyl, and z is an integer of 1 to 5.
Among others, those siloxanes of the formula: 
wherein Me is methyl and z is an integer of 1 to 5 are most preferred because of ease of synthesis and rapid film formation (a short tack-free time).
Usually, the siloxane (c) is added in an amount of 0.01 to 10 parts, preferably 0.1 to 5 parts, and more preferably 0.5 to 4 parts by weight per 100 parts by weight of the organopolysiloxane (a). With less than 0.01 part of component (c), a longer time is taken for film formation (a longer tack-free time). More than 10 parts of component (c) extremely quickens film formation or shortens the tack-free time, resulting in difficulty of working.
If necessary, numerous fillers may be blended in the inventive composition. Suitable fillers include finely divided silica, silica aerogel, precipitated silica, diatomaceous earth; metal oxides such as iron oxide, zinc oxide, titanium oxide, and aluminum oxide; metal carbonates such as calcium carbonate, magnesium carbonate and zinc carbonate; asbestos, glass wool, carbon black, fine mica, fused silica powder, and powdered synthetic resins such as polystyrene, polyvinyl chloride, and polypropylene. The fillers may be blended in any desired amount as long as this does not compromise the objects of the invention. It is preferred that the fillers be dried to remove water before use. If desired, the composition may further include pigments, dyes, antidegradants, antioxidants, antistatic agents, flame retardants (e.g., antimony oxide and chlorinated paraffin), and thermal conductivity modifiers (e.g., boron nitride).
Other additives that can be incorporated in the inventive composition include thixotropic agents such as polyether, mildew-proofing agents, antibacterial agents, and adhesive aids, examples of which are aminosilanes such as xcex3-aminopropyltriethoxysilane and 3-(2-aminoethylamino)propyltrimethoxysilane, and epoxysilanes such as xcex3-glycidoxypropyltrimethoxysilane and xcex2-(3,4-epoxycyclo-hexyl)ethyltrimethoxysilane. In particular, the aminosilane is added in an amount of preferably 0.1 to 10 parts, more preferably 0.2 to 5 parts by weight per 100 parts by weight of the organopolysiloxane (a).
If necessary, hydrocarbon solvents such as toluene and petroleum ether, ketones and esters may be added as the diluent.
The inventive composition is obtained by intimately mixing the essential components (a) to (c) and optionally, fillers, various additives, diluents and the like in a dry atmosphere.
The inventive composition remains stable in the sealed state, but when exposed to air, rapidly cures with airborne moisture and firmly adheres to numerous substrates. The composition thus finds effective use in a wide variety of applications as a sealing agent, coating agent, adhesive, coating, water repellent, fiber or fabric treating agent, and parting agent.